This invention relates to a hologram, a method of manufacturing a hologram, and a coherent beam scanner.
A hologram is more specifically called a holographic zone plate. The hologram is for use in, among others, a coherent beam scanner. A hologram has a constant that corresponds to the focal length of a thin lens and is also named a focal length F. As is the case with a thin lens, the focal length F is dependent on the wavelength of a coherent beam incident on the hologram.
A laser beam is usually used in a coherent beam scanner as a coherent beam. The coherent beam scanner is therefore referred to often as a laser beam scanner and will be so named in the following. As will later be described more in detail with reference to a few of a multiplicity of figures of the accompanying drawing, a hologram is used in a laser beam scanner as a diffraction grating for diffracting a laser beam illuminating a limited area of the hologram along an axis of illumination. The hologram produces a diffracted laser beam. The angle of diffraction varies when the hologram is linearly moved relative to the axis of illumination. The diffracted beam therefore scans a plane, herein called an output plane, as a small light spot or scan spot along a straight line. The length of the line is dependent on the maximum angle of deflection of the diffracted beam relative to the axis of illumination and on the ratio of the distance between the hologram and the output plane to the focal length F of the hologram for the illuminating beam. The ratio will herein be referred to as a factor of magnification M.
Inasmuch as linearly moved in a laser beam scanner, a hologram is rectangular in general outline so that the limited area may move along the length of the hologram during the linear movement. It is known to manufacture such a hologram by recording on a hologram recording medium, such as a photographic plate record, a number of interference fringes formed by interference of two coherent beams. Fringes of higher-order interference are recorded on both end portions of the hologram with fringes of lower-order interference recorded on a central portion.
A sophisticated hologram for use typically in a laser beam scanner, is manufactured by the use of a spherical-wave and a collimated beam. When the limited area covers that portion of the hologram on which higher-order interference fringes are recorded, aberrations become serious. The highest order of interference up to which it is possible to record the interference fringes for practical purposes, is therefore restricted. In other words, it has been impossible to manufacture a long hologram for practical applications and to widen the limited area. Furthermore, it has been necessary for practical uses to restrict the angle of deflection of the diffracted beam only to a few degrees in angle on either side of the axis of illumination.
Wai-Hon Lee proposed a phase plate for correcting the aberrations. The proposal is contributed to "Applied Optics," Volume 16, No. 5 (May 1977), pages 1392-1399, under the title of "Holographic Grating Scanners with Aberration Corrections." The phase plate is necessary only during manufacture of the holograms. It is, however, not so easy to manufacture the phase plate with a precision high enough to carry out the aberration corrections to a satisfactory extent.
An improved hologram is revealed in U.S. patent application Ser. No. 9,720 now U.S. Pat. No. 4,299,437, filed Feb. 5, 1979, by Yuzo Ono, the present applicant and assignor to the instant assignee. It is possible for the improved hologram to raise the highest order of interference to four times that for a sophisticated hologram and thereby to provide a hologram about twice as long as the sophisticated hologram. The improved hologram is, however, still insufficient in aberration corrections. It has been objectionable to raise the factor of magnification M up to four or more. The improved hologram is therefore inadequate for use in an image recorder where it is necessary to make the light spot scan a long line with the dimension of the spot strictly restricted.